1. Field of Invention
This invention relates generally to methods for treating continuously moving webs. In particular, it is concerned with tunnel drying of webs previously coated with a liquid medium. It combines the effects of gaseous fluid impingement which treats the web and optionally supports it with high frequency energy input to the web, preferably radio frequency (R.F.), which enhances the treatment.
2. Prior Art
High speed air impingement dryers are widely used in industry to dry a variety of web products such as paper, photographic films, coated fabrics, et cetera. In their more advanced form, the air jets from the nozzle bars are used also to float and position the web as it moves along the drying path thus avoiding mechanical contact with the web and reducing web tension build up through the dryer. In this form, such dryers provide generally good service and drying speed. Like all air drying systems, however, they are subject to several inherent limitations:
(a) The product must be over-dried in part to insure that any wetter or heavier coated areas are fully dried before exiting the dryer to other in-line processing steps or a windup. This is particularly troublesome when the process web contains anomalously heavier coated areas such as edges, coater skips, or splashes. In many cases, such anomalies determine the maximum process speed rather than the normal product drying. PA1 (b) To obtain higher drying speeds for a given dryer path length, the operator's only routes are to increase the air jet velocities or the air temperature. Neither can be increased indefinitely since excessive values of either may damage the coating or the base web. PA1 (c) As the drying speed is pushed higher, the problem of "skinning" of an initially wet coating surface becomes more pronounced. When all or most of the drying energy is transferred through the surface, a moisture gradient is set up in the coating thickness. This causes the surface to become drier than the bulk of the coating and thus lose mobility in the critical early drying phases. This prevents the surface tension in the coating from acting beneficially to smooth out surface irregularities that occur naturally in any coating process. The action described has been observed by most people in seeing brush marks gradually disappear from a slowly drying varnish coating. PA1 (a) In the typical case where the wet coating is the principal R.F. lossy energy receptor, dielectric heating will provides a compensating action to level the drying of coating anomalies across and along the web. This is a result of the energy being selectively absorbed proportional to the amount of the dielectrically lossy solvent locally present in the web. PA1 (b) Because the dielectric energy is liberated directly in the bulk of the web or coating, a more uniform moisture gradient in the thickness direction is achieved. This reduces the "skinning" effect described earlier and usually results in improved surface smoothness in coated webs. PA1 (c) A higher rate of energy input for faster drying can often be achieved because the dielectric coupling bypasses the limitations of conventional convective heat transfer. Thus, air velocities or temperatures of the air impingement dryer which may be excessively high for the product are avoided. PA1 (a) A dielectric dryer design for a given application generally turns out to be a custom engineering and process development effort and is, therefore, usually time consuming and costly. PA1 (b) With conventional dielectric dryers, it is difficult to predict the energy input profile along the dryer path and just as difficult to adjust it after the process is put in operation. For sensitive products, this represents a very high technical risk for the plant operator who is attempting to obtain some of the the inherent benefits of dielectric heating. As a result, most process operators opt for a system using only air impingement drying since it can be engineered faster and more predictably. PA1 (a) The unitized assemblies, if used as direct, bolt-in replacements for conventional nozzle bars, will permit simple, low risk development of critical process/product parameters by direct on-line tests. The units can be quickly installed in various numbers or positions and evaluated. So called "A/B" test comparisons may be obtained with "air only" drying simply by turning off the electrical power. If for some reason, the supplemental R.F. energy input causes detrimental effects, normal production can be resumed simply by turning off the electrical power or re-bolting the conventional nozzle bars in place. PA1 (b) By providing integral R.F. generators in each R.F./Nozzle bar module, the assemblies can be utilized over very long tunnel lengths in any positional arrangement without encountering the technical problems and costs attendant to bussing all the bars from a remote common R.F. generator. PA1 (c) The use of low power individual generators in each module provides two useful characteristics that were not available in the previous art. First, the power level of each assembly may be made different or remotely adjustable so that a specific R.F. power input profile can be established along the dryer path length. This can be particularly useful in the processing of sensitive products. Secondly, as each module power supply is limited in its total output, it is unlikely an anomalous heavy coating defect can cause a catastrophic energy input wherein all the available power of a large common R.F. generator is drawn into one small portion of the web. PA1 (d) The electrical circuit features which maintain the transferred power substantially constant independent of typical gap variations will provide the dryer operator a wider process latitude than has been available with previous art. By reducing the effects of web tension, product changes et cetera which can all affect the gap variations, it should be possible to improve process efficiency and quality control. PA1 (e) By incorporating the fringing field electrode elements in the proximate face of the nozzle bar several important advantages are obtained over previous art.
Many of the difficulties in air or radiant dryers described above can be alleviated by introducing dielectric heating energy into the web during the drying process. To be beneficial, this additional input need only represent a portion of the total energy needed to evaporate the solvent from the web. In most practical applications, a good volume of air impingement flow onto the web must be maintained to dilute and carry off the evaporated solvent. The general characteristics of dielectric drying which make it useful in process drying have been well covered in the literature and patent art. Briefly, these factors are:
Because the benefits of dielectric drying previously described are generally well known in the process drying industries, much effort has been expended over the years towards developing improved dielectric dryers both in the radio frequency and microwave areas. Some efforts have been quite successful but many have been abandoned for a variety of reasons. Those familiar with the art will generally agree the following difficulties are common:
Most of these difficulties stem from the design of the conventional radio frequency web dryer. Typically it consists of a ladder-like array of alternating electrically "hot" and grounded electrodes which establish a fringing electric field to intercept the proximate process web. The electrodes are bussed together on heavy R.F. conductor systems to a common radio frequency power generator. R.F. generator powers in the 10 kilowatt to 50 kilowatt range are fairly common in the industry. In this arrangement, it is difficult to extend the applicator length or remote the common generator more than about one-quarter wavelength of the operating frequency because of the voltage standing wave effects that are encountered. In such an arrangement, the voltage level across each electrode pair and hence the R.F. electric field is substantially the same throughout the whole array. The overall level can be adjusted from the common generator or through various circuit coupling means known in the art. With this arrangement, the difficulty in predicting or controlling the power input to the web along its drying path through the electrode system stems from two major effects. First, the local energy coupling to the web will vary strongly as the inverse of the gap between the electrode pair and the web. Thus, if the planarity or positioning of the web is less than perfectly controllable, the local energy input will also be uncontrollable. The second factor has to do with the complex nature of the dielectric loss factor of the material which is the receptor for the energy. In R.F. systems, this is usually a partially conductive solvent containing ionic solutes. In microwave systems, additional mechanisms come into play such as polar molecule coupling. The amount of energy locally transferred depends simultaneously on the local conditions of solvent quantity, its solute concentration, and its temperature. All these factors are varying along the dryer path in a complex and interdependent manner.
What is needed by the drying industry is an efficient approach to combining the best features of air impingement drying with the best features of dielectric drying. When used in combination, a synergism results to produce a drying system superior to either approach used alone. Both mediums contribute to the total energy transfer to the web. The R.F. contributes to leveling of coating anomalies while the air impingement carries off evaporated solvent and helps maintain the coating temperature nearer the dew point rather than its boiling point.
There have been some efforts in the industry to achieve this goal and get around the engineering and process problems associated with combining air impingement and dielectric drying. One such approach is described in U.S. Pat. No. 4,257,167 issued to H. C. Grassmann. In that approach, individual air impingement nozzle bars of an approximately conventional design are made to act as alternate polarity electrode bars of a stray field R.F. coupler. The active fringing R.F. field is established between the separate nozzle bars. This generally leads to inefficient dielectric energy coupling because the optimum nozzle bar spacing is usually too large to establish an optimum R.F. field. Grassman partially overcomes this by showing optional satellite electrode bars added between the nozzle bars. The entire set of nozzle bars acting as electrodes is driven from a common R.F. power generator as in a conventional R.F. stray field ladder electrode.
Although the arrangement described by Grassman will provide a route to achieving a potentially useful combination of air impingement and dielectric drying, it is still subject to all the engineering and process problems ascribed earlier to dielectric dryers. These include the problems of distributing substantial amount of R.F. power from a remote generator over the length of a tunnel dryer which might extend hundreds of feet. The arrangement also precludes much versatility in experimenting with the optimum number and placement of R.F. heating zones in an existing long tunnel dryer and establishing a controllable power transfer profile along that length.
Others have attempted to achieve combined air flow and dielectric drying by utilizing microwave power sources. Such microwave applicators, typically utilizing serpentine wave guide sections experience great difficulty in providing controlled distribution of energy input both across and along the web in the processing zone.
3. Definitions
Nozzle Bar, as referred to herein, means a structure, usually elongated, disposed transversely to the path of the moving web and in close proximity thereto. It provides jets of gas through one or more slot or hole orifices to impinge on a proximate web. This impinging flow is typically used to heat, condition, or dry the web. In addition, the kinetic energy of the gas flow may be directed so as to create a zone of pressure higher than ambient to provide mechanical positioning and/or support of the moving web.
Air, as referred to herein, means any gaseous fluid capable of transporting sensible or latent heat to or from the web and usually is capable of transporting any solvent vapors released from the web to collection points away from the processing area. In a typical application, the gas is air of a controlled temperature and humidity.
Proximate, as referred to herein, means the region between the nozzle bar and the web within the area projected by the individual nozzle bar structure on the web. Depending on the portion of the discussion, it may be used in conjunction with the web surface, working surface of the nozzle bar, or the space between these two. This is the region wherein the impingement gas jets provide the bulk of their heat transfer action and, also, provide any pressure support for web positioning if that feature is included in the design.
Radio Frequency (or R.F.), as referred to herein, means an electrical voltage or current whose polarity is reversing periodically with time. A generally accepted frequency of reversal for R.F. is approximately 0.5 Megahertz to approximately 500 Megahertz. For industrial heating applications there are legal and technical preferences for operating at one of the ISM (Industrial, Scientific, and Medical) bands allocated in Part 18 of the Federal Communications Commission regulations.